1. Field of the Invention
This invention relates to a process for preparing telechelic structures.
2. Description of Related Art
Telechelic structures are polymers whose chain ends contain functional groups, eg. --OH, --SH, --NH.sub.2, etc. They may serve as building polyesters, etc., polymers functionalized at both chain ends being particularly important.
The anionic polymerization of, for example, butadiene or isoprene with in particular bifunctional dilithium initiators in hydrocarbons is a particularly suitable process for the synthesis of diene telechelic structures. It gives living polymers having a narrow molecular weight distribution and suitable microstructure, since high proportions of the dienes are incorporated in the 1,4 configuration. Suitable initiators are also known (for example EP 305909 (1988), EP 363659 (1989) or EP 405327 (1990)). The living polymers exhibiting bifunctional growth are then converted into the telechelic structures by polymer-analogous reaction with suitable functionalizing reagents.
Particularly desirable telechelic structures are those having terminal primary or secondary hydroxyl groups, which are prepared in a preferred process (cf. for example U.S. Pat. No. 3,786,116 which is incorporated herein by reference) by reacting the living polymers with epoxides, eg. ethylene oxide or propylene oxide.
Very stable associates of the chain ends are formed by the conversion of the (less polar) organolithium terminal groups into strongly polar lithium alcoholate terminal groups, leading to a dramatic increase in viscosity or, in the case of functionalization which results in more than 40-50% of bifunctional species, to gelling of the reaction material owing to physical crosslinking. Because of the high reaction rate, gelling begins almost immediately after the addition of the terminating reagent, the uniform incorporation of which by mixing is thus prevented. The consequences are incomplete reaction and a lower quality of the telechelic structures, and possibly also blocking of the stirrer or even damage to the plant parts. It is true that termination can be realized at low concentrations; however, such a procedure is uneconomical. There was therefore a need for a process which permits termination with formation of polar terminal groups without these hindrances and at high polymer concentration.
It is an object of the present invention to provide such a process.